1. Technical Field of the Invention
The present invention relates to transflective liquid crystal devices. In particular, the present invention relates to multi-gap type liquid crystal devices in which the thickness of a liquid crystal layer between a transmissive display region and a reflective display region in each pixel is changed to a suitable value.
2. Description of the Related Art
In various types of liquid crystal devices, those which are capable of displaying both in a transmissive mode and in a reflective mode are referred to as transflective liquid crystal devices and are used in all scenes.
As shown in (A), (B), and (C) of FIG. 24, a transflective liquid crystal device has a transparent first substrate 10 having a first transparent electrode 11, a transparent second substrate 20 having a second transparent electrode 21 opposing the first transparent electrode 11, and a TN (twisted nematic) liquid crystal layer 5 held between the first substrate 10 and the second substrate 20. The first substrate 10 has a light-reflecting layer 4 defining a reflective display region 31 in a pixel region 3 where the first transparent electrode 11 opposes the second transparent electrode 21. An opening 40 in the light-reflecting layer 4 defines a transmissive display region 32. The outer surfaces of the first substrate 10 and the second substrate 20 have polarizers 41 and 42, respectively, and a backlight device 7 opposes the polarizer 41.
In the liquid crystal device 1 having such a structure, light emitted from the backlight device 7 and entering the transmissive display region 32 enters the liquid crystal layer 5 through the first substrate 10. The light is modulated in the liquid crystal layer 5 and emitted from the second substrate 20 to serve as transmissive display light to display images (transmissive mode), as indicated by Arrow L1.
On the other hand, light entering the reflective display region 31 through the second substrate 20 reaches the reflecting layer 4 through the liquid crystal layer 5. The light is reflected at the reflecting layer 4 and emitted from the second substrate 20 through the liquid crystal layer 5 to serve as reflective display light to display images (reflective mode), as indicated by Arrow L2.
In the first substrate 10, the reflective display region 31 and the transmissive display region 32 are provided with a reflective-display color filter 81 and a transmissive-display color filter 82, respectively, so that color images can be displayed.
In such optical modulation, if the twist angle of liquid crystal is set small, the change in polarization is expressed as a function of the product of the difference Δn between refractive indexes and the thickness d of the liquid crystal layer 5 (retardation Δn·d). Therefore, by appropriately setting this value, the visibility of images can be improved. In the transflective liquid crystal device 1, however, while the transmissive display light passes through the liquid crystal layer 5 only once, the reflective display light passes through the liquid crystal layer 5 twice. Therefore, it is difficult to optimize the retardations Δn·d of both the transmissive display light and the reflective display light. Specifically, when the thickness d of the liquid crystal layer 5 is set so that the visibility in the reflective mode is improved, images in the transmissive mode are degraded. In contrast, when the thickness d of the liquid crystal layer 5 is set so that the visibility in the transmissive mode is improved, images in the reflective mode are degraded.
Japanese Unexamined Patent Application Publication 11-242226 discloses a structure in which the thickness d of the liquid crystal layer 5 in the reflective display region 31 is set smaller than the thickness d of the liquid crystal layer 5 in the transmissive display region 32. This structure is referred to as a multi-gap type and is realized, for example, by providing a thickness-adjusting layer 6 having an opening 61 formed in the region corresponding to the transmissive display region 32, under the first transparent electrode 11 and above the light-reflecting layer 4, as shown in (A), (B), and (C) of FIG. 24. Specifically, since the thickness d of the liquid crystal layer 5 in the transmissive display region 32 is larger than that in the reflective display region 31 by the thickness of the thickness-adjusting layer 6, the retardations Δn·d of both the transmissive display light and the reflective display light can be optimized. In order to adjust the thickness d of the liquid crystal layer 5, the thickness of the thickness-adjusting layer 6 formed must be large. Such a thick layer is formed of a photosensitive resin or the like.
When the thickness-adjusting layer 6 is formed of a photosensitive resin, photolithography is used. However, the thickness-adjusting layer 6 inevitably has slopes 60 diverging upward at the boundary between the reflective display region 31 and the transmissive display region 32 because of the low exposure accuracy and side etching caused by development during the photolithography. As a result, the thickness d of the liquid crystal layer 5 changes continuously at the boundary between the reflective display region 31 and the transmissive display region 32, and the retardation Δn·d continuously changes accordingly. Also, the initial orientation of liquid crystal molecules contained in the liquid crystal layer 5 is determined by alignment layers 12 and 22 formed on the innermost surfaces of the first substrate 10 and the second substrate 20. However, since the alignment strength of the alignment layer 12 acts in tilted directions in the slopes 60, the orientation of the liquid crystal molecules is disordered in these areas, as schematically shown in FIG. 25, and thus disclination occurs.
For example, if the known liquid crystal device 1 is designed for use in a normally white mode, the entire image should be displayed black while an electric field is applied. However, light leaks from the region corresponding to the slopes 60 and, thus, display failures, such as contrast degradation, occur. FIG. 26(A) shows the results of a simulation for estimating the distribution of the reflected light intensity for each rubbing direction from the reflective display region 31 to the transmissive display region 32 when the entire image is displayed black. As shown in FIG. 26(A), light leaks at the boundary between the reflective display region 31 and the transmissive display region 32. This continuously variable light leakage is caused by an unsuitable retardation Δn·d and the sharp peak of the light leakage is caused by an alignment failure of liquid crystal. FIG. 26(B) shows the results of a simulation for estimating the distribution of the transmitted light intensity for each rubbing direction from the reflective display region 31 to the transmissive display region 32 when the entire image is displayed black. As shown in FIG. 26(B), light leaks at the boundary between the reflective display region 31 and the transmissive display region 32. This continuously variable light leakage is also caused by an unsuitable retardation Δn·d and the sharp peak of the light leakage is caused by an alignment failure of liquid crystal. The leakage level of the transmitted light is notably lower than that of the reflected light.
Accordingly, one object of the present invention is to provide a multi-gap type liquid crystal device in which the thickness of the liquid crystal layer is changed to proper values from the transmissive display region to the reflective display region in one pixel region and to provide an electronic apparatus using the liquid crystal device. In the structure of the liquid crystal device, even if the retardation is not proper or the orientation of the liquid crystal molecules is not aligned, high-quality images can still be displayed.